From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BC221C433DF for ; Thu, 18 Jun 2020 13:27:05 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 98280207D8 for ; Thu, 18 Jun 2020 13:27:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 98280207D8 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jluZA-0003Sm-EB; Thu, 18 Jun 2020 13:26:52 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jluZ9-0003Sg-8U for xen-devel@lists.xenproject.org; Thu, 18 Jun 2020 13:26:51 +0000 X-Inumbo-ID: 5dbce122-b167-11ea-8496-bc764e2007e4 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 5dbce122-b167-11ea-8496-bc764e2007e4; Thu, 18 Jun 2020 13:26:50 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 8A166AD2A; Thu, 18 Jun 2020 13:26:48 +0000 (UTC) Subject: Re: [PATCH v2 for-4.14] x86/vmx: use P2M_ALLOC in vmx_load_pdptrs instead of P2M_UNSHARE To: Tamas K Lengyel , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <3555e16baa9e1909dbefaaab04330694135c9021.1592410065.git.tamas.lengyel@intel.com> <20200618125205.GO735@Air-de-Roger> From: Jan Beulich Message-ID: Date: Thu, 18 Jun 2020 15:26:50 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Kevin Tian , Tamas K Lengyel , Wei Liu , Paul Durrant , Andrew Cooper , Jun Nakajima , Xen-devel Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 18.06.2020 15:00, Tamas K Lengyel wrote: > On Thu, Jun 18, 2020 at 6:52 AM Roger Pau Monné wrote: >> >> On Thu, Jun 18, 2020 at 02:46:24PM +0200, Jan Beulich wrote: >>> On 18.06.2020 14:39, Tamas K Lengyel wrote: >>>> On Thu, Jun 18, 2020 at 12:31 AM Jan Beulich wrote: >>>>> >>>>> On 17.06.2020 18:19, Tamas K Lengyel wrote: >>>>>> While forking VMs running a small RTOS system (Zephyr) a Xen crash has been >>>>>> observed due to a mm-lock order violation while copying the HVM CPU context >>>>>> from the parent. This issue has been identified to be due to >>>>>> hap_update_paging_modes first getting a lock on the gfn using get_gfn. This >>>>>> call also creates a shared entry in the fork's memory map for the cr3 gfn. The >>>>>> function later calls hap_update_cr3 while holding the paging_lock, which >>>>>> results in the lock-order violation in vmx_load_pdptrs when it tries to unshare >>>>>> the above entry when it grabs the page with the P2M_UNSHARE flag set. >>>>>> >>>>>> Since vmx_load_pdptrs only reads from the page its usage of P2M_UNSHARE was >>>>>> unnecessary to start with. Using P2M_ALLOC is the appropriate flag to ensure >>>>>> the p2m is properly populated and to avoid the lock-order violation we >>>>>> observed. >>>>> >>>>> Using P2M_ALLOC is not going to address the original problem though >>>>> afaict: You may hit the mem_sharing_fork_page() path that way, and >>>>> via nominate_page() => __grab_shared_page() => mem_sharing_page_lock() >>>>> you'd run into a lock order violation again. >>>> >>>> Note that the nominate_page you see in that path is for the parent VM. >>>> The paging lock is not taken for the parent VM thus nominate_page >>>> succeeds without any issues any time fork_page is called. There is no >>>> nominate_page called for the client domain as there is nothing to >>>> nominate when plugging a hole. >>> >>> But that's still a lock order issue then, isn't it? Just one that >>> the machinery can't detect / assert upon. >> >> Yes, mm lock ordering doesn't differentiate between domains, and the >> current lock order on the pCPU is based on the last lock taken >> (regardless of the domain it belongs to). > > I see, makes sense. In that case the issue is avoided purely due to > get_gfn being called that happens before the paging_lock is taken. > That would have to be the way-to-go on other paths leading to > vmx_load_pdptrs as well but since all other paths leading there do it > without the paging lock being taken there aren't any more adjustments > necessary right now that I can see. If this is indeed the case, then I guess all that's needed is a further extended / refined commit message in v3. Jan